An affordable housing proposal on the ballot would provide relief for low- and middle-income residents who can't buy a home in San Francisco - but it would force heavy cuts to city services like health care, critics say.

Proposition B would set aside 2.5 cents of every $100 of the city's property taxes - or about $30 million a year - for affordable housing every year for the next 15 years. Proponents, including eight members of the Board of Supervisors, say the measure would guarantee funding for an issue that is one of the city's most critical, especially in the current economic climate.

"This will add a level of certainty to affordable housing production. That's important because affordable housing is not a single-year issue," said Supervisor Chris Daly. "It makes sense to have a longer-term, more stable revenue source of funding so that we can actually address the affordable housing crisis in San Francisco."

But Mayor Gavin Newsom says the measure is unnecessary - the supervisors already have the power to set aside funding for affordable housing - and would further strangle the city's budget at a time when San Francisco is facing a possible $250 million deficit in 2009.

Newsom said more than 90 percent of the city's property tax revenues are already earmarked for specific programs, leaving little leeway when it comes to balancing the budget and leading to cuts in unprotected services. Since the city's social services budgets are the largest, they tend to bear the brunt of the cuts, he said.

"This measure is ballot-box budgeting at its worst. There's nothing stopping the Board of Supervisors from redirecting money for more affordable housing," Newsom said. "Why didn't they redirect money to affordable housing this year if they care so much about it?"

He added that supervisors who support the measure have not suggested how the city will make up for the money that would be redirected to affordable housing.

"You've got to balance the budget. Where are they going to get the money?" Newsom said.

The measure is necessary in part, proponents say, because the current funding sources - primarily federal grants - don't provide for San Francisco's affordable housing needs. The city has a shortage of housing for families, in particular, and needs affordable homes for not just the poorest residents, but even people at the median income level.

The measure would require that at least half of all new affordable housing units have two bedrooms or more, and that 40 percent of new units go to very low-income households that make less than 30 percent of the city's median income.

"We've gotten bonds, we've gotten federal money, but that money is becoming increasingly difficult to access - and, worse, is being earmarked for narrower and narrower sets of the population," said longtime city housing advocate Calvin Welch, who is working with the Prop. B campaign.

In addition to putting money aside, the measure would require the Board of Supervisors to create an affordable housing plan every three years and hold annual meetings to review the plan.

One of the arguments against the measure is that the city already spends a substantial amount of money, mostly from outside sources, on affordable housing. But Welch said those programs are so widespread that it's nearly impossible to determine how useful they actually are.

"The city spends somewhere around $250 million a year from all sources on all programs, but if you wanted to try to track that, you'd have to go to four different commission hearings and two different departments," Welch said. "This measure would require a single budget be placed before the board in one time and in one place, so the people of San Francisco could see what's happening."

Proposition B

What it is: Measure requires the city to take about $30 million out of the budget each year and use the cash to build affordable housing over the next 15 years.

Arguments in support: The city doesn't put enough money into affordable housing, and a stable source of funding would help solve San Francisco's housing shortage for low-income residents, including families with children.

Arguments against: The supervisors already have the flexibility to spend that money on affordable housing, and tying up all that money could hamstring the city in a budget crisis. Setting the money aside means less cash for other efforts, such as providing health care. The city is already facing social services cuts again next year because of a projected $250 million deficit.

Who opposes it: Mayor Gavin Newsom, three city supervisors, San Francisco Apartment Association, Coalition for Better Housing, Chamber of Commerce, Coalition for San Francisco Neighborhoods, San Francisco Association of Realtors.